Tissue culture methods are powerful tools which provide means to vegetatively propagate genetically superior plants of many species. Such methods have also been useful in a number of special applications, including rejuvenation of mature plant material, virus elimination, and genetic transformation. Of the various components of a tissue culture method, the basal nutrient medium is one of the most important factors influencing the success of culturing plant material in vitro (Gamborg et al. 1976) .
As the nutrient medium is an essential part of tissue culture methods, it is not surprising that a number of different aqueous nutrient medium formulations have been taught in published reports on conifer tissue cultures. Many of these reports describe the use of aqueous nutrient media initially developed for use with other species. For example, MS (Murashige and Skoog 1962), SH (Schenk and Hildebrandt 1972), and GD (Gresshoff and Doy 1972) media were developed for use with angiosperms, but have been used for the tissue culture of Pinus ponderosa Dougl. ex Law. (Tuskan et al. 1990, Lin et al. 1991), Pinus caribaea Morlet (Webb and Santiago 1983), and Pinus taeda L. (Neuman et al. 1992). In various conifer tissue culture reports, several previously published medium formulations were screened and the one eliciting the best responses was chosen for subsequent use (Bornman 1983, Abdullah et al. 1985); while in other reports, various dilutions of published media were tested (Mott and Amerson 1981, Harry et al. 1994). Despite numerous attempts, the efforts to develop tissue culture methods for utilization with loblolly pine (Pinus taeda L.), have met with only limited success.
A number of aqueous basal nutrient media which are commonly used in tissue culture methods for various conifer species were evaluated specifically for use with loblolly pine. These basal media included GD.sub.mod medium (Mehra-Palta et al. 1978), LMG.sub.20 medium (Mott et al. 1986), LP medium (Aitken-Christie et al. 1987), SH medium (Schenk and Hildebrandt 1972), and DCR medium (Becwar et al. 1990).
The GD.sub.mod, LMG.sub.20, LP, and SH media were tested for axillary shoot micropropagation and shoot culturing using several loblolly pine families. In this evaluation, significant problems were associated with the use of each of these basal media. Of the four media, SH medium was the least successful. Only a low percentage of the explants initially cultured on SH medium resulted in the development of axillary shoots, and very few of these axillary shoots lived long enough to give rise to additional new shoots. Many shoots on SH medium suffered from necrosis of the shoot tip, which eventually led to the death of the entire shoot. As few genotypes survived on SH medium for an entire year, this medium had the lowest percentage of surviving explants and the lowest overall production of axillary shoots of the four media tested. Survival was better on LP medium, but certainly not optimal. Only 43% of the explants responded with axillary shoots. Many of these resulting shoots also suffered from necrosis of the shoot tip, which eventually led to the death of the entire shoot. Survival was highest on GD.sub.mod and LMG.sub.20 media (64% and 69% respectively) but the resulting shoots exhibited undesirable mature characteristics. The overall rate of in vitro shoot growth and the response to root-induction treatments were significantly lower for shoots from GD and LMG.sub.20 media than for shoots from LP medium. To summarize: when utilized in tissue culture methods for loblolly pine shoots, none of these published nutrient media achieved the desired results of high survival rates, good shoot production, and enhancement of juvenile characteristics (such as rapid sustained shoot growth and high rooting potential).
Several published basal media were also evaluated for their effect on somatic embryogenesis using several loblolly pine families (Chesick and Becwar 1992). Immature megagametophytes containing zygotic embryos at a developmental stage previously determined to be responsive (Becwar et al. 1990) were cultured, and data on the extruding embryogenic tissue were collected. Overall, DCR and SH media resulted in highest frequencies of extrusion and proliferation of embryogenic tissue. Despite this ranking, the frequency of embryogenic tissue extrusion and proliferation was low. Initial extrusion percentages averaged 30% for SH and 28% for DCR. Moreover, the percentage of cultures which continued to proliferate declined rapidly, and by nine weeks averaged only 11% for SH and 7% for DCR. To summarize: none of these published nutrient media resulted in the survival of a large percentage of responding cultures for loblolly pine somatic embryogenesis.
Therefore, an object of the present invention is to provide an effective aqueous basal nutrient media for in vitro tissue cultures of loblolly pine.